In recent years, as a method for forming solder resist layers on printed wiring boards for consumer use and industrial use, a developable liquid solder resist composition with excellent resolution and dimensional accuracy has been widely used, instead of a screen printing method, in order to increase the density of wiring on the printed wiring board.
An epoxy compound is sometimes contained in a solder resist composition in order for the solder resist composition to gain thermosettability in addition to photocurability. In patent literature 1, a hydroquinone epoxy compound is disclosed as a kind of epoxy compound which can be contained in a solder resist composition. A coating formed by applying the solder resist composition containing the hydroquinone epoxy compound on a substrate has good alkali developability and low tackiness. A solder resist layer formed by curing the coating has good flexibility, heat crack resistance, and heat resistance.
However, there exists a problem that, when the solder resist layer is formed with the solder resist composition containing a powdery hydroquinone epoxy compound, glossiness and surface smoothness of the solder resist layer decrease. If the surface smoothness of the solder resist layer is low, slip marks are likely to be caused on the solder resist layer when transporting a printed wiring board including the solder resist layer. Also, when the solder resist layer and metal such as copper rub against each other, the metal is likely to be scraped and the metal powder generated from the scraped metal is likely to adhere to the solder resist layer. Due to this, deficiency in surface appearance of the solder resist layer tends to occur.
In order to improve the surface smoothness of the solder resist layer, the hydroquinone epoxy compound should be heated and melted in advance, and then the solder resist composition is obtained by kneading ingredients of the solder resist composition including the melted hydroquinone epoxy compound so that the hydroquinone epoxy compound and components other than the hydroquinone epoxy compound are mixed. In such process, however, equipment as well as time and effort for heating and melting are required due to the hydroquinone epoxy compound having a melting point of about 138 to 145° C., resulting in decreased productivity of the solder resist compound. In addition, since the hydroquinone epoxy compound has high crystallinity, even if the hydroquinone epoxy compound is heated in advance so that the hydroquinone epoxy compound is contained in the solder resist composition in melted state, cooling the solder resist composition is likely to cause crystallization and precipitation of the hydroquinone epoxy compound. Therefore, it is difficult to apply and dry the solder resist composition with the hydroquinone epoxy compound being melted.